A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus may be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which may be referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern may be transferred on (part of) the substrate (e.g. a glass plate), e.g. via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
Instead of a circuit pattern, the patterning means may be used to generate other patterns, for example a color filter pattern, or a matrix of dots. Instead of a mask, the patterning device may comprise a patterning array that comprises an array of individually controllable elements. An advantage of such a system compared to a mask-based system is that the pattern can be changed more quickly and for less cost.
A flat panel display substrate may be rectangular in shape. Lithographic apparatus designed to expose a substrate of this type may provide an exposure region which covers a full width of the rectangular substrate, or which covers a portion of the width (for example half of the width). The substrate may be scanned underneath the exposure region, whilst the mask or reticle is synchronously scanned through the projection beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure may be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate may be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
In the manufacture of devices using a so-called maskless lithographic apparatus (namely one in which the mask has been replaced by a programmable patterning array), a pulsed radiation source may be used. Between pulses of radiation, the pattern programmed on the programmable patterning array may be updated and the substrate advanced relative to the patterned radiation beam such that a series of patterned images (which will typically be different from each other) are projected on to the substrate. In such an arrangement, in order to ensure that the overall quality of the complete image formed is of an acceptable level, it is desirable to ensure that the dose of radiation within successive pulses are as similar as possible in order to minimise differences between images projected on to the substrate by different pulses. For example, if the radiation dose of a first pulse of the radiation system is greater than the radiation dose of a second pulse of the radiation system, the critical dimension (CD) of an image projected on to the substrate by the first pulse may differ from the critical dimension of an image projected on to the substrate by the second pulse, even if the intended patterns are identical.
Unfortunately, conventionally known radiation sources that provide a pulsed beam of radiation with the required radiation wavelength and frequency of pulsing, are not able to provide the required pulse to pulse dose stability. It has therefore previously been proposed to provide a mechanism for adjusting the radiation dose within pulses of radiation after the pulses have been generated by the radiation source in order to reduce the radiation dose variation between pulses. However, for a maskless lithographic apparatus to be used for commercial manufacture of devices, the radiation source must provide as large a radiation dose as possible within each pulse of radiation. This is because the larger the dose of radiation within a pulse, the larger the area on a substrate on which a patterned image can be projected. Accordingly, the larger the radiation dose within a pulse of radiation, the faster the entire image to be projected on the substrate can be completed, leading to a shorter processing time for the substrate and a more efficient and more cost-effective lithographic apparatus. However, the presently known mechanisms for adjusting the radiation dose within a pulse of radiation are not suitable for use with radiation sources that generate radiation pulses with sufficiently high doses for a commercial lithographic apparatus.